Apple might be readying its largest iPhones ever. Regardless of how you feel about that, it could mean one unequivocally crappy thing for all of us: The extinction of great, casual one-handed games.

The usual trickle of pre-release rumors suggests the iPhone 6 could come in two sizes: 4.7 inches and 5.5 inches. Enlarging the form factor ostensibly would help Apple better compete with already-huge Android handsets. But it also would jeopardize the iPhone's promise of comfortable one-handed use -- and that's where the problem arrives with respect to games.

If designers can't be sure we can grasp our phones with one hand, with our thumbs free to tap most of the screen, will they still spend their time making games that demand these conditions? If one-handed phones die out, why wouldn't we assume one-handed games will follow?

Bennett Foddy, the gamemaker behind the Flash sensation QWOP and its iPhone incarnation, recently sparked the discussion on Twitter. A mega-iPhone would be a "massive mistake" on Apple's part, he wrote. "I don't really care as a consumer, mind you. But it matters to me as an iOS game developer, where it's just going to be a design nightmare."

How Fragmentation Affects Game Design

On one hand, the problem bigger iPhones pose to well-crafted games is simply one of fragmentation. It's something that's been a mounting challenge to exacting designers and developers since the initial iPhone.

"At the most basic level, good game design is concerned with the micro details of interactions," Foddy explains. "How does it feel if I put this button 1.2 inches from the edge of the screen instead of 1.1? Does it feel nice and natural to swipe my finger in this particular arc?"

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In the early days of smartphones, when there was a single touchscreen form factor to develop for, it was easy to work out those tiny details. Today, Foddy says, you just can't achieve that level of tactile fine-tuning.

To make money, designers must put their games on as many screens as possible. As a result, many settle for making the experience "adequate" on devices large and small, Android and iOS. Effectively splitting the iPhone market into three sizes (including legacy 4-inchers) would only add to this problem.

But larger iPhones could mean an even more significant shift in the mobile games landscape. While you can find one-handed games for your gigantic Android phone, the majority of truly novel, thoughtfully designed portrait-orientation games arrive at least initially and often exclusively on the iPhone.

Think Threes, Ridiculous Fishing, Letterpress, Device 6. These are the types of games most at risk amidst the great smartphone embiggening.

The Gaming Use-Case That's at Risk

It helps to remember that smartphones gave rise to a fundamentally new type of game, making way for the titles that have distracted us from our line-waiting miseries for the past several years. "Knowing that someone can touch most of the screen with one hand, you can design apps or games that live in the busiest parts of a person's life," Foddy says. "On the train, or at the bank, I can play Flappy Bird or Threes or Letterpress, but I can't play a 3-D shooter."

But if the next generation of iPhones are indeed 4.7 inches and 5.5 inches diagonally across, as has been rumored, it would mean that a significant number of iPhone users won't be able to comfortably touch the whole screen with one hand. It makes those "busiest part of your life" games a much trickier proposition—and potentially bad business.

"If I make an app in portrait orientation, I have to assume half my customers will be holding it with two hands," Foddy says. And if he wanted to make a game explicitly designed for one hand, "I basically have to give away a large portion of the market."

Asher Vollmer, co-creator of the hit portrait-orientation numbers puzzler Threes, thinks Foddy's concerns are legitimate. "One of the biggest complaints I get on Threes is that the retry button is too far away," he says. "It's in the top left of the screen. I can just barely get away with this because you don't restart games too often, but if I released a game that required area-specific touch controls I would be in a lot of trouble with newer, gigantic phones."

Is Awkward Gaming Inevitable?

Vollmer does point out that portrait games like Threes and Temple Run might ultimately be safe because their main gameplay controls work anywhere on the screen. But in a mondo-phone future, games with area-specific controls like Cut the Rope will have a more difficult route ahead.

It's possible games will adapt, moving their controls to a thumb-friendly interaction zone in the bottom-right (or bottom-left) corner of the screen, but that isn't the most elegant use of a huge touchscreen, nor will it work with every type of title. (Plus, the problem with larger displays isn't just thumb-reach; it's that one-handed operation of any sort becomes fraught at a certain size. You get people pinching the side with their palms, accidentally triggering taps on the screen, or else grasping them two-fisted like a Game Boy. All this uncertainty adds challenge for game designers.)

While a wave of bigger iPhones could be a death knell for a certain type of one-handed, casual game, there's evidence that these sorts of games are already on the way out. Just look at the bestseller charts for Android Play and the iOS App Store. As of writing, there was exactly one portrait-mode game to be found among the top twenty bestsellers on both stores.

It's a glossy update of Tetris for the iPhone, an old game that just happens to have lived in a long, tall rectangle all its life. By comparison, App Store records for the same week in 2010 show that a full half of the top-selling paid games for the iPhone were portrait-orientation.

So why the dearth of popular portrait titles on the charts today? It might be that we're just increasingly coming to think of smartphones as sideways, two-handed gaming devices. As phones have become more powerful, games have become more cinematic, and cinema always looks better in widescreen. Plus, we now have other portrait-mode diversions like Twitter and Instagram to fill those idle moments in line and on the subway. The addicting pleasure of "pull-to-refresh" even lends some of those apps a game-like element of their own.

Looking at those App Store charts, Foddy can't help but see evidence that mobile gaming has turned on its side.

"The days of DoodleJump and SpellTower and Letterpress are gone," he says. "Everything is in two-handed landscape mode, a mode for people who are sitting down or lying in bed, and devoting all their attention and manual capability to the game." In other words: Bigger, better, brighter phones have lead us to bigger, better brighter games.

The demise of the one-handed game becomes fascinating when you take this slightly longer view. For a brief spell, the advent of a touchscreen device that fit comfortably in one hand gave birth to an entirely new type of diversion. These were tiny things, just as easy to put down as they were to pick up.

We didn't have to carve out time to play them; they lived in nooks and crannies of down time during our day. But with the rise of less graspable phones and the abundance of time-wasters we now have for them, the conditions that allowed for those games to exist may well be evaporating.